docs: def rrtmg_lw_gen_source(ext, build_dir):
      '''Add RRTMG_LW fortran source if Fortran 90 compiler available,
      if no compiler is found do not try to build the extension.'''
      thispath = config.local_path
      module_src = []
      for item in modules:
          fullname = join(thispath,'rrtmg_lw_v4.85','gcm_model','modules',item)
          module_src.append(fullname)
      for item in src:
          if item in mod_src:
              fullname = join(thispath,'sourcemods',item)
          else:
              fullname = join(thispath,'rrtmg_lw_v4.85','gcm_model','src',item)
          module_src.append(fullname)
      sourcelist = [join(thispath, '_rrtmg_lw.pyf'),
                    join(thispath, 'Driver.f90')]
      try:
          config.have_f90c()
          return module_src + sourcelist
      except:
          print('No Fortran 90 compiler found, not building RRTMG_LW extension!')
          return None